One known mechanism used in such a tool is the crank and connecting rod mechanism in which a first member to be reciprocated is pivotably connected to one end of a connecting rod whose other end is rotatably mounted on a crank, the axis of rotation of the crank being perpendicular to the path of reciprocation. This mechanism is quite satisfactory for a wide range of applications but in some circumstances the fact that the motion of the reciprocated member is not Simple Harmonic Motion is a disadvantage. Also the mechanism is necessarily quite long, especially if motion approximating to Simple Harmonic Motion is to be achieved, and this can in some circumstances be a disadvantage. One known alternative to the connecting rod is the "Scotch Yoke" mechanism. In this mechanism the crank pin is mounted in a slot or guide channel on the member to be reciprocated, the slot or channel extending perpendicular to the path of reciprocation. This mechanism has the advantage that it is much more compact than the connecting rod mechanism and that the motion of the reciprocated member is Simple Harmonic Motion. However, in order to avoid excessive wear of the coupling between the crank pin and the slot or guide the construction of the mechanism must be quite sophisticated and consequently this "Scotch Yoke" mechanism is unduly expensive for certain applications.